1. Technical Field
This invention relates generally to transcutaneous energy transmissions systems. In particular, the invention relates to the use of two coils to inductively transmit radio frequency power across an intact skin boundary for use by a device implanted under the skin, or otherwise within the body of, a living being.
2. State of the Art
Transcutaneous energy transfer systems (TETS) have been used to provide power for a number of implanted devices such as: low power prosthetic devices; cochlear implants, muscle stimulators, bone growth stimulators and stimulators of the visual cortex, and higher power devices; total artificial hearts, and ventricular assist devices. The inductively coupled coils of a TETS, one implanted under the skin, the other resting on the surface of the skin, permit electrical interaction between the implanted device and external circuits through intact skin, and bring about the transfer of power from the external circuit to the internal circuit avoiding penetration of the skin by electrical conductors. FIG. 1 illustrates a vertical cross section through an exemplary prior art TETS as implanted and in position for use.
The interaction between the coils requires alternating current; usually a frequency between 100 kHz and 1 MHz is chosen. In order to improve the efficiency with which power is delivered, the coils are incorporated into either series or parallel resonant circuits by connecting them to capacitors. The resonant circuits can be tuned with a natural resonant frequency less than, equal to, or greater than the operating frequency, as described by the U.S. Pat. Nos. 4,441,210 and 5,070,535 to Hochmair et al. and U.S. Pat. No. 4,679,560 to Galbraith. In the following discussion, the efficiency with which power is transferred to the internal coil will be referred to as link efficiency. This quantity is related to the overall efficiency of the TETS by the expression: EQU .eta..sub.overall =(.eta..sub.DC to RF)(.eta..sub.link)(.eta..sub.RF to DC) (1)
Thus, the overall efficiency is equal to the product of three sequential process efficiencies.